Introduzione a Java Server Pages Lucidi tratti da www.corewebprogramming.com The Need for JSP • With servlets, it is easy to – Read form data – Read HTTP request headers – Set HTTP status codes and response headers – Use cookies and session tracking – Share data among servlets – Remember data between requests • Much more difficult is to – Use those println statements to generate HTML – Maintain that HTML The JSP Framework • Idea: Turn Code and HTML inside out – Use regular HTML for most of page – Mark servlet code with special tags – Entire JSP page gets translated into a servlet (once), and servlet is what actually gets invoked (for each request) • Example: – Suppose the following JSP is in OrderConfirmation.jsp Thanks for ordering <em><%= request.getParameter("title") %></em> – URL http://host/OrderConfirmation.jsp?title=Core+Web+Programmi ng – Result: 1
Benefits of JSP • Although JSP technically can't do anything servlets can't do, JSP makes it easier to: – Write HTML – Read and maintain the HTML • JSP makes it possible to: – Use standard HTML development tools – Have different members of your team do the HTML layout and the programming • JSP encourages you to – Separate the (Java) code that creates the content from the (HTML) code that presents it Example <BODY> <H2>JSP Expressions</H2> <UL> <LI>Current time: <%= new java.util.Date() %> <LI>Your hostname: <%= request.getRemoteHost() %> <LI>Your session ID: <%= session.getId() %> <LI>The <CODE>testParam</CODE> form parameter: <%= request.getParameter("testParam") %> </UL> </BODY> </HTML> Example Result • With default setup, if location was – http://almaak.usc.edu:8821/examples/jsp/Chapt er20/ Expressions.jsp?testParam=some+data 2
Most Common Misunderstanding: Forgetting JSP is Server-Side Technology • Very common question – I can’t do such and such with HTML. Will JSP let me do it? • Why doesn't this question make sense? – JSP runs entirely on server – It doesn’t change content the browser can handle • Similar questions – How do I put an applet in a JSP page? Answer: send an <APPLET…> tag to the client – How do I put an image in a JSP page? Answer: send an <IMG …> tag to the client – How do I use JavaScript/Acrobat/Shockwave/Etc? Answer: send the appropriate HTML tags 2nd Most Common Misunderstanding: Translation/Request Time Confusion • What happens at page translation time? – JSP constructs get translated into servlet code • What happens at request time? – Servlet code gets executed. No interpretation of JSP occurs at request time. The original JSP page is ignored at request time; only the servlet that resulted from it is used • When does page translation occur? – Typically, the first time JSP page is accessed after it is modified. This should never happen to real user (developers should test all JSP pages they install). – Page translation does not occur for each request Types of Scripting Elements • Expressions – Format: <%= java expression %> – Evaluated and inserted into the servlet’s output. I.e., results in something like out.println(expression) • Scriptlets – Format: <% code %> – Inserted verbatim into the servlet’s _jspService method (called by service) • Declarations – Format: <%! code %> – Inserted verbatim into the body of the servlet class, outside of any existing methods. Globals for the class. 3
Expressions: <%= Java Expression %> • Result – Expression evaluated, converted to String, and placed into HTML page at the place it occurred in JSP page – That is, expression placed in _jspService inside out.print • Examples Current time: <%= new java.util.Date() %> Your hostname: <%= request.getRemoteHost() %> Note: the variable request above is the same as the parameter to the doGet or doPost servlet routines. More about these variables later. JSP/Servlet Correspondence • Original JSP <H1>A Random Number</H1> <%= Math.random() %> • Possible resulting servlet code public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setContentType("text/html"); HttpSession session = request.getSession(true); JspWriter out = response.getWriter(); out.println("<H1>A Random Number</H1>"); out.println(Math.random()); ... } Example Using JSP Expressions <BODY> <H2>JSP Expressions</H2> <UL> <LI>Current time: <%= new java.util.Date() %> <LI>Your hostname: <%= request.getRemoteHost() %> <LI>Your session ID: <%= session.getId() %> <LI>The <CODE>testParam</CODE> form parameter: <%= request.getParameter("testParam") %> </UL> </BODY> 4
Predefined Variables • request – The HttpServletRequest (1st arg to doGet) • response – The HttpServletResponse (2nd arg to doGet) • session – The HttpSession associated with the request • out – The stream (of type JspWriter) used to send output to the client associated with the response Scriptlets : <% Java Code %> • Result – Code is inserted verbatim into servlet's _jspService • Two Examples <% String queryData = request.getQueryString(); out.println("Attached GET data: " + queryData); %> Or: Attached GET data: <%= request.getQueryString() %> <% response.setContentType("text/plain"); %> JSP/Servlet Correspondence • Original JSP <%= foo() %> <% bar(); %> • Possible resulting servlet code public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setContentType("text/html"); HttpSession session = request.getSession(true); JspWriter out = response.getWriter(); out.println(foo()); bar(); ... } 5
Example Using JSP Scriptlets <HTML> <HEAD> <TITLE>Color Testing</TITLE> </HEAD> <% String bgColor = request.getParameter("bgColor"); boolean hasExplicitColor; if (bgColor != null) { hasExplicitColor = true; } else { hasExplicitColor = false; bgColor = "WHITE"; } %> <BODY BGCOLOR="<%= bgColor %>"> <% if (hasExplicitColor) out.println(…); %> … JSP Scriptlets: Results Declarations: <%! Java Declarations %> • Result – Code is inserted verbatim into servlet's class definition, outside of any existing methods • Examples – <%! private int someField = 5; %> – <%! private void someMethod(...) {...} %> 6
JSP/Servlet Correspondence • Original JSP <%! private String randomHeading() { return("<H2>" + Math.random() + "</H2>"); } %> <H1>Some Heading</H1> <%= randomHeading() %> JSP/Servlet Correspondence • Possible resulting servlet code public class xxxx implements HttpJspPage { private String randomHeading() { return("<H2>" + Math.random() + "</H2>"); } public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setContentType("text/html"); HttpSession session = request.getSession(true); JspWriter out = response.getWriter(); out.println("<H1>Some Heading</H1>"); out.println(randomHeading()); ... } Example Using JSP Declarations <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>JSP Declarations</TITLE> <LINK REL=STYLESHEET HREF="JSP-Styles.css" TYPE="text/css"> </HEAD> <BODY> <H1>JSP Declarations</H1> <%! private int accessCount = 0; %> <H2>Accesses to page since server reboot: <%= ++accessCount %></H2> </BODY> </HTML> 7
JSP Declarations: Result • After 15 total visits by an arbitrary number of different clients The import Attribute • Format – <%@ page import="package.class" %> – <%@ page import="package.class1,...,package.classN" %> • Purpose – Generate import statements at top of servlet Example of import Attribute ... <BODY> <H2>The import Attribute</H2> <%-- JSP page directive --%> <%@ page import="java.util.*,cwp.*" %> <%-- JSP Declaration --%> <%! private String randomID() { int num = (int)(Math.random()*10000000.0); return("id" + num); } private final String NO_VALUE = "<I>No Value</I>"; %> 8
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